Organic nanotubes are tubular structures having hollow pores with an inner diameter of 7 to 200 nm that are formed by self-assembly of sugars, amino acids and other organic molecules. Various attempts have been made thus far to find their applications as nanocapsules (See, Non-Patent Document 1 below, which is incorporated herein by reference in its entirety). For example, organic nanotubes have been disclosed that are functionalized on their inner or outer surfaces in order to control the size, i.e., the outer and inner diameter, of the organic nanotubes (See, Non-Patent Document 2 and Patent Documents 1 and 2 below, each of which is incorporated herein by reference in its entirety). Further, organic nanotubes capable of encapsulating a protein in the small cavity within the tube or carrying biomaterials such as DNA or drugs on the outer surface of the tube and subsequently releasing them have been disclosed (See, Non-Patent Documents 3 to 5 below, each of which is incorporated herein by reference in its entirety).
Specifically, the present inventors have developed bicephalic lipids that have different hydrophilic moieties at both ends of a hydrophobic alkyl chain. The present inventors have reported the generation of asymmetric organic nanotubes with their inner and outer surfaces covered with different functional groups by self-assembly of such lipids (See, Non-Patent Documents 6 and 7 and Patent Document 1 below, each of which is incorporated herein by reference in its entirety). In some cases, this asymmetric characteristic has been utilized to achieve efficient encapsulation or sustained release of a biomaterial or a drug that has an opposite charge to that of the inner surface of the tube, by making use of the electrostatic attraction between the biomaterial or drug and the inner surface of the tube (See, Patent Document 3 and Non-Patent Document 6 below, each of which is incorporated herein by reference in its entirety). It has also been reported that the controlled drug release or protein refolding through encapsulation and release of a denatured protein can be achieved by introducing hydrophobic functional groups or the like selectively onto the inner surface of the tube (See, Patent Document 4, Non-Patent Documents 7 and 8 below, each of which is incorporated herein by reference in its entirety).
On the other hand, a drug delivery hydrogel has been reported that provides a varying sustained release rate of a protein or DNA from agarose gel depending on the guest size. The hydrogel is obtained by providing microtubes having an outer diameter in the order of about 0.5 micrometers and encapsulating a protein or DNA with a correspondingly large molecular weight, and subsequently dispersing the microtubes in an aqueous solution of agarose gel, followed by gelation of the solution (See, Non-Patent Documents 9 and 10, each of which is incorporated herein by reference in its entirety).